The controlled amplification of electromagnetic waves has many uses. For example, intelligence may be conveyed along a wave by amplifying electromagnetic wave characteristics so that the amplified wave can be perceived after transmission through a medium at a distance. Power amplifiers are commonly used in the communication field to increase the power of a modulated RF (radio frequency) signal which is then delivered to an antenna for transmission through the atmosphere. Two types of power amplifiers are current-source power amplifiers, in which a transistor acts as a current source, and switch-mode power amplifiers in which a transistor acts as a switch.
The output power of a switch-mode power amplifier is proportional to the resistance of the switch, however the switch may be implemented. One example is a metal oxide semiconductor (MOS) transistor acting in triode as a switched resistor. In many applications, especially cellular communication systems, it is important to precisely control the output power of the power amplifier. In order to accomplish this, the resistance of a switching transistor may be regulated by varying the gate-source voltage across the transistor (i.e., to start or stop current flow). However, a typical switching transistor is “inductively loaded,” which means that it receives a supply voltage via an RF choke inductor. This arrangement leads to particularly large voltage variations on the drain of the switching transistor during signal changes. For example, in an idealized Class F switch-mode power amplifier, the voltage at the drain of a switching transistor will be 2× the supply voltage switching from “closed” to “open.” The instantaneous voltage variations and accompanying electric fields can be particularly large relative to the maximum allowed voltages for sub-micron CMOS transistors commonly employed, for example, as one of the final stages of a large signal switch-mode power amplifier.
At the sub-micron level in CMOS technologies, switch-mode power amplifier performance and reliability may be negatively affected by instantaneous voltage variations due to “hot electron” effects. “Hot electrons” are individual holes or electrons which are highly accelerated due to high local electric fields. When the kinetic energy of these carriers exceeds the barrier height of the silicon gate insulator of a switching transistor, they may “jump” the barrier and enter the insulator. Over time, the effects of the accumulation of carriers in the silicon gate insulator may degrade the threshold voltage, VT, drain current and/or output power of the switching transistor.
What is needed is a technique for regulating the instantaneous voltage variations across a switching transistor used for power amplification.